Bronze alloys containing iron



Sept. 19, 1961 c. H. HANNON 3,000,733

BRONZE ALLOYS CONTAINING IRON Filed Oct. 1. 1959 PERCENT 0/ /20 3,000,733 BRONZE ALLOYS CONTAINING IRON Cyril H. Harmon, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Filed Oct. 1, 1959, Ser. No. 843,709 4 Claims. (Cl. 75-462) The present invention relates to copper-base alloys and more particularly to silicon-aluminum bronze alloys having improved corrosion resistant properties.

Known bronze alloys containing aluminum and silicon have been found to have excellent forging and machining qualities as well as superior resistance to ordinary types of corrosion such as caused by marine atmospheres. Ordinary corrosion is generally considered to be chemical corrosion wherein the corrosive medium chemically dissolves the metal on which it acts and causes a gradual thinning of the metal. However, while bronze alloys are highly desirable for many industrial hardware applications, they have the disadvantage, in common with copperbase alloys in general, of being susceptible to a type of defect known in the metallurgical arts as stress corroslon.

The kind of corrosion met with in stress-corrosion is electrochemical corrosion caused by the existence in the alloy structure of more or less continuous zones that are anodic to contiguous portions of the alloy. The presence of such anodic portions causes a corrosive medium to set up a galvanic cell in which such medium .is the electrolyte and the contiguous portions of difierent electrochemical potential are the electrodes, the anodic zone or portion dissolving into such electrolyte by galvanic action, with the result that the surface of the alloy subjected to the medium becomes locally pitted as distinguished from being gradually thinned by chemical solution.

While the surface of an alloy having these contiguous relatively anodic and cathodic portions is subjected to the action of a corrosive medium, minute fissures or pits are initiated at such surface due to this electro-chemical or galvanic action, and when residual stress from prior mechanical working and/ or high tensile stresses resulting from tensile or torque loading of the article made of the alloy are present, these small fissures increase the magnitude of local stresses at the fissures and that increases the depth of the fissures. This increase in the depth of the fissures exposes fresh metal at the bottoms thereof, the thus exposed fresh metal being consumed by the electrolytic action with consequent further increase in the magnitude of the stresses at the fissures, and as the action progresses, the alloy ultimately cracks or otherwise fails. The stress corrosion as thus described leads ultimately to a form of rupture called stress-corrosion cracking, sometimes also referred to as season cracking.

It is an object of the present invention to provide bronze alloys which have improved resistance to stress corrosion.

It is a specific object of the invention to provide siliconaluminum bronze alloys which are characterized by high tensile strength, good resistance to chemical corrosion, and excellent working behavior, and which are particularly marked by high resistance to stress corrosion effects.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying claims.

A type of bronze alloy to which the present invention is particularly applicable is an alloy known under the trademark Duronze, which has a typical composition of about 91% copper, 7% aluminum, and 2% silicon. In accordance withthe invention, alloy compositions of the above type are modified by the addition thereto of a small but critical amount of iron, such that the modified alloy contains about 3 to 6% of iron. The bronze alloy rates Etent provided by the invention for obtaining the above objects has a composition of about 85 to 89.2% copper, 6 to 8% aluminum, 1.8 to 2.2% silicon and about 3 to 6% In tests conducted in connection with the present invention, two series of copper base alloys with varying percentages of iron were prepared for subjecting to stress corrosion conditions. One series included a number of high brass alloys having varying amounts of iron therein. The other series had a basic composition corresponding to that of the above-mentioned Duronze with different iron percentages therein. In preparing the alloy samples for test, ingots of the respective alloy compositions were hot extruded, hot swaged, and subsequently hot forged into eye bolts. A subsequent operation involved the cold piercing of the hole in the eye section. The eye-bolts were machined with /8 diameter screw threads having 16 US. standard screw threads per inch, and were assembled in steel fixtures and clamped by nuts on .their threaded portions so as to apply measured torque loads, in accordance with standard procedure. ,To determine" the susceptibility of these alloys to stress corrosion fracture, the respective eye-bolt samples, while subjected to torque loads in the above manner, were exposed in "closed vessels to the fumes of a 10% aqueous solution of ammonium hydroxide. As recognized by those skilled in the art, copper base alloys are susceptible to stress corrosion failure in the presence of atmospheres containing even traces of ammonia or ammonia-containing compounds, such atmospheres being found in industrial and other areas of widely varying type.

The following table shows the approximate composition in percent by weight of the high brass alloys tested:

Table I Alloy-No. 1 on 1 s1 Fe Zn 'Pu As shown above, varying amounts of iron from .04 to 7.4% were incorporated in the bronze alloys. More than the 7.4% iron maximum shown was found to produce excessive brittleness in the alloy and higher percentages, consequently, were not used in the samples to be tested.

With the high brass alloy eye-bolts of Table I subjected to a torque load of of their ultimate torque strength and exposed to a 10% ammonium hydroxide atmosphere as described above, all of these eye-bolts cracked within four days.

In the case of the bronze alloy eye-bolts of Table II,

Table III Iron Content Days The considerable improvement of the iron-modified bronze alloys over the high brass alloys in their resistance to stress corrosion conditions is quite evident from the above data.

Even more remarkable is the surprising degree of improvement in those bronze alloys which had iron added thereto in the range of about 36%.

The figure in the drawing is a graphical illustration of the results thus obtained from the iron-modified bronze alloys, the percent iron content in the alloysbeing-plotted against days to failure. The critical nature of iron content in the range of about 3-6% in producing a sharp rise in the resistance of the bronze alloys to atmospheres conducive to stress corrosion is clearly evident from the graph.

' The properties of the improved bronze alloy as described above make this material particularly applicable for use in electric cable pressure terminals, fixtures for electric power transmission lines, turnbuckle and other eye-bolts, and other articles which in use are subjected to high tensile loads and the prolonged electro-chernical corrosive effect of industrial atmospheres. other wrought articles will as a result of the invention be characterized by superior resistance to failure from stress- These and 4 corrosion and chemical corrosion, as well as having excellent working properties.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in-theart without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent 'variations as come Within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A bronze alloy characterized by excellent working characteristics and high resistance to chemical and stress corrosion comprising approximately 6 to 8% aluminum, 1.8 to 2.2% silicon, 3 to 6% iron, and the balance copper.

2. A bronze alloy characterized by excellent working characteristics and high resistance to chemical and stress corrosion comprising approximately 7.3% aluminum, 1.9% silicon, 3.6% iron, and the balance copper.

3. A wrought article characterized by high resistance to chemical and stress corrosion and having excellent working characteristics, said article being made of an alloy comprising about 6 to 8% aluminum, 1.8 to 2.2% silicon, 3 to 6% iron, and the balance copper. A

4. A wrought article characterized by high resistance to chemical and stress corrosion and having excellent working characteristics, said article being made of an alloy comprising about 7.3% aluminum, 1.9% silicon; 3.6% iron, and the balance copper.

References Cited in the file of this patent UNITED STATES PATENTS 1,838,632 Pacz Dec. 29, 1931 2,744,822 Briggs et al. May 8, 1956 2,789,900 Hannon Apr. 23, 1957 2,870,051 Klement Jan. 20, 1959 FOREIGN PATENTS 703,304 Germany Mar. 6, 1941 

1. A BRONZE ALLOY CHARACTIZED BY EXELLENT WORKING CHARACTERISTICS AND HIGH RESISTANCE TO CHEMICAL AND STRESS CORROSION COMPRISING APPROXIMATELY 6 TO 8% ALUMINUM, 1.8 TO 2.2% SILICON, 3 TO 6% IRON, AND THE BALANCE COOPER. 